Conceptual Foundations

The parallels between embryonic stages and the 'scale of beings' had already been contemplated in pre-Darwinian times, and the foundation of a scientific theory of evolution was significantly influenced by embry-ological arguments. Darwin called embryology 'by far the strongest single class of facts in favour of a change of form', and his first sketches of a phylogenetic tree seem to have been inspired by tree-like renderings of embryological differences between species (Richards 1992). Much of...

Urbisexuality the evolution of bilaterian germ cell specification and reproductive systems

A key focus of evolutionary developmental biology (evo-devo) in recent years has been to elucidate the evolution of developmental mechanisms as a means to reconstructing the hypothetical last common ancestors of various clades. Prominent among such reconstructions have been proposals as to the nature of the mysterious Urbilateria, originally defined as the last common ancestor (LCA) of the extant Bilateria (Ecdysozoa, Lophotrochozoa and Deuterostomia) (De Robertis and Sasai 1996, Kimmel 1996)....

The molecular biology underlying developmental evolution

Stephen Jay Gould opens the Prospectus of his influential Ontogeny and Phytogeny (Gould 1977) with the following quotation from Van Valen (1973) 'A plausible argument could be made that evolution is the control of development by ecology. Oddly, neither area has figured importantly in evolutionary theory since Darwin, who contributed much to each. This is being slowly repaired for ecology . . . but development is still neglected.' As accurate as these comments may have been in 1977, today, 30...

Ontogeny of the spiralian brain

Entoprocta Larve

Spiral cleavage is a characteristic feature of several protostomian taxa, sometimes united as Spiralia (Dohle 1996), but its presence in a number of these groups has been debated. This could be the result of too vague definitions, so I will emphasise here the presence of both a spiral pattern with shifting direction of the spindles in the early cleavages and a cell lineage including prototroch cells (trochoblasts) differentiating from cells along the border between first and second micromere...

References

Problems with the interpretation of developmental sequences. Systematic Zoology 34, 46-58. Arthur, W. 2000. The concept of developmental reprogramming and the quest for an inclusive theory of evolutionary mechanisms. Evolution & Development 2,49-57. Breidbach, O. & Ghiselin, M. T. 2007. Evolution and development Past, present, and future. Theory in Biosciences 125, 157-171. Chen, J., Braun, A., Waloszek, D., Peng, Q.-Q.& Maas, A. 2004. Lower Cambrian yolk-pyramid...

Many roads lead to Rome different ways to construct a nematode

Many Roads Roma

Einhard schierenberg and jens schulze It has been well established that considerable differences exist in the developmental pattern among animal taxa, for instance with respect to how blastomeres perform their early cleavages, how they acquire different fates or how symmetry is formed (Gilbert and Raunio 1997). Even among relatively closely related species, for instance within sea urchins or tunicates, impressive differences can be found in the pattern of development ( Jeffery et al. 1999, Raff...

Rag Rag Rag Rag Rag

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Info

Figure 9.1 Schematic summary of vulva formation in C. elegans. A, During the L1 stage, the 12 ventral epidermal cells P(1-12).p are equally distributed between pharynx and rectum. B, P(1,2,9-11).p fuse with the hypodermal syncytium hyp7 (F, white ovals). P(3-8).p form the vulva equivalence group and adopt one of three alternative cell fates. P6.p has a 1 fate (black oval), and P(5,7).p have a 2 fate (grey ovals). P(3,4,8).p have a 3 fate and remain epidermal (dotted ovals). The anchor cell (AC,...

Evodevo methods and materials

If evo-devo is a discipline in its own right, is there a distinctive set of biological systems and methods of investigation through which it is currently advancing Although evo-devo probably does not rely upon specific tools of analysis unknown in other fields of biological research, because of its particular relationships to both evolutionary and developmental biology evo-devo exhibits a specific combination of model systems and research tools. In other words, to use a fashionable term in...

Acoela

Figure 12.3 An abbreviated phylogenetic tree depicting some metazoan clades with, above the nodes, the number of new miRNAs appearing at each cladogenetic event. The number of different miRNAs in Acoela is low (7 miRNAs) whereas that of 'Platyhelminthes' (Catenulida + Rhabdito-phora) is similar (33 miRNAs) to those of other lophotrochozoans like annelids and molluscs. This supports previous work suggesting the poly-phyly of Platyhelminthes and the basal position of Acoelomorpha (Ruiz-Trillo et...

Evolution of neurogenesis in arthropods

Several alternative hypotheses have been suggested that support various phylogenetic groupings of the individual euarthropod taxa, the chelicerates, myriapods, crustaceans and insects. The Tetraconata hypothesis suggests a sister-group relationship of insects and crustaceans in contrast to the traditional monophyletic grouping of myria-pods and insects, the Tracheata or Atelocerata (see references in Stollewerk and Chipman 2006). The Mandibulata hypothesis suggests a clade consisting of...

Thoughts and speculations on the ancestral arthropod segmentation pathway

In the past decade or so, there has been a significant increase in the available data on the developmental mechanisms underlying the process of segmentation in a wide range of arthropod taxa. This large body of data makes it possible to attempt, albeit cautiously, a comparative analysis of the various aspects of the segmentation process, and to try to find which of its features and components may have been present in the arthropod common ancestor. A recent review (Peel et al. 2005) covers much...

Basal euarthropod development a fossilbased perspective

Subphylum Trilobites Structure

Nigel c. hughes, joachim t. haug and dieter waloszek The morphological gap between Euarthropoda (the crown group that contains all extant arthropods) and living arthropod-like animals such as onychophorans, tardigrades and pentastomids is bridged by a number of fossils known primarily from rocks some 520 to 490 million years old (e.g. Fuxianhuia, Chengjiangocaris, Shankouia, see Figure 15.1 cf. Waloszek et al. 2005). These centimetre-scale fossil animals illuminate critical steps in early...

Stem Crown

Figure 12.1 Conflicting phylogenies and scenarios on the nature and origin of the Last Common Ancestor (LCA) of the Bilateria, also featuring the extent of stem and crown groups. A, The complex Urbilateria scenario features a large, complex ancestor bearing most characters of present-day bilaterians (characters 1-9, and eventually characters 10-11 in Table 12.1). This ancestor originated from either an adult (polyp) or a larval radial cnidarian (archicoelomate theory, originally proposed by...

Nkl

However, the Msx4 gene (now called MSX2P Genbank accession NR_002307) that is linked to the HOXB cluster in humans is a processed pseudogene, and has been dismissed as useless for reconstructing ancestral linkage patterns (Castro and Holland 2003). It is still intriguing, however, that of all of the places in the genome that an Msx processed pseudogene could jump into, it should happen to land right next to a Hox cluster, within 10 Mb. The Mega-homeobox...

Imhh

Ancestral genes + non-canonical seq. evol. Figure 10.1 The diversity of the Hox-linked and NK-linked genes was already established early in animal evolution, but were the genes linked in a Mega-homeobox cluster or not Conventionally a distinction has been made between Hox-like and NK-like (NKL) genes in the ANTP-class. However, the boundary between these two groups is very fuzzy, with poorly resolved molecular phylogenetic trees of homeodomain sequences. The coloration used here instead...

Lbx

TLX2 (HOX11L1, NCX, Enx, Tlx2), TLX3 (HOX11L2, RNX) NK1slou NKX1-1 (HSPX153, SAX1, NKX1.1), NKX1-2 (NKX1.2, SAX2) ladybird early (lbe), ladybird late (lbl) C15 (93Bal, clawless, Hox11-311, Ect5) slouch (slou) (paired-like9, S59, NK-1) AmphiNK1a & b, NveNK1 NveSLOU PduNK1

Mega Evolution In Zoology

Evolution of the gene network underlying wing polyphenism in ants. Science 297, 249-252. Amundson, R. 2005. The Changing Role of the Embryo in Evolutionary Thought Roots of Evo-Devo. Cambridge Cambridge University Press. Ankeny, R. A. 2001. Model organisms as models understanding the 'Lingua Franca' of the human genome project. Philosophy of Science 68, S251-S261. Arthur, W. 1997. The Origin of Animal Body Plans. A Study in Evolutionary Developmental Biology....

Genetic Information

Figure 5.4 Possible representations of the multidimensional space controlling gene regulation in eukaryotic cells. A, Individual genetic elements (black circles) interact with many other elements through positive and negative interactions (activation and repression arrows, respectively) quantitative aspects of these interactions are represented by variable thicknesses (strength of the interaction) and lengths (duration of the interaction) of the interaction arrows. B, As a concept diagram,...

Plants are used to having identity crises

Rolf rutishauser, valentin grob and evelin pfeifer Macroscopic nature is never really anomalous. Abnormalities, like other exceptional cases, at least show incontestably, what the plants can do. However, regardless of how much faith one has in anatomical definitions, they should not be taken as more than a means of communication prior to subsequent genetic analysis. Truth, except as a figure of speech, does not exist in empirical science. Our green and living world is a continuum in space and...

The Impact Of Evodevo On Evolutionary Theory

The conceptual framework of evo-devo arose as a response to the incompleteness of the neo-Darwinian synthesis (Callebaut et al. 2007). The paradigm of the synthesis was based on the correlation of phenotypic character variation with statistical changes of gene frequencies in populations. Adaptive change as a population genetic event was the explanandum. The paradigm of evo-devo, by contrast, represents a causal-mechanistic approach towards the explanation of phenotypic change in evolution. Here...

On comparisons and causes in evolutionary developmental biology

Denn mit dem Warum der Dinge kommt niemand zu Ende. Die Ursachen alles Geschehens gleichen den D nenkulissen am Meere eine ist immer der anderen vorgelagert, und das Weil, bei dem sich ruhen lie e, liegt im Unendlichen. For once you begin with the Why you can never get to the end. It is like the dunes by the sea, where behind each dune lies still another and the Because where you might come to final rest lies somewhere in infinity. Thomas Mann, Joseph und seine Br der Comparison is fundamental...

Unravelling body plan and axial evolution in the Bilateria with molecular phylogenetic markers

Jaume baguna, pere martinez, jordi paps and marta riutort setting the problem The emergence of dramatic morphological differences (disparity) and the ensuing bewildering increase in the number of species (diversity) documented in the fossil record at key stages of animal and plant evolution have defied, and still defy, the explanatory powers of Darwin's theory of evolution by natural selection. Among the best examples that have captured the imagination of the layman and the interest of scores...

Evolving body features

'Distinctive features', 'key apomorphies', 'novelties', 'innovations'. All these terms point to the fact that some characters in an organism are 'special' in some respect, and these have attracted the most interest from biologists and challenged the explanatory capacity of evolutionary theories. These are characters that emerged as new features during the evolution of a lineage, or that are believed to be the principal cause of a successful phyletic radiation. 'Evolution is tinkering' and in...

Arthropod appendages a prime example for the evolution of morphological diversity and innovation

Nikola-michael prpic and wim g. m. damen The morphology of the appendages of the arthropods has been adapted to a large number of life styles that is virtually unparalleled in any other organ in the Metazoa. Different appendage types exist e.g. for walking, swimming, jumping, prey-capture, chewing, biting, mating, egg-laying, breathing in air, fresh water and salt water, and sensory perception see Figure 20.1 for examples . Very specialised appendage types exist for specialised modes of life...

A pragmatic approach for selecting evodevo model species in amniotes

Amniote Evolutionary Tree

Athanasia c. tzika and michel c. milinkovitch One major classical justification of using a model metazoan species for experimentation has been that discoveries of biological phenomena in that species could be extrapolated to other multicellular species. Because the chances that this extrapolation is valid in humans depend on the phylogenetic distance between humans and the model species, many researchers have somewhat sacrificed the major benefits of small size, short generation time and ease...

Prospects of evodevo for linking pattern and process in the evolution of morphospace

Evolutionary developmental biology or evo-devo will make crucial contributions over the coming years to exploring the occupancy of mor-phospace. Why do species show the patterns of diversity and disparity they do, and to what extent do such patterns reflect the ways in which phenotypic variation is generated as well as the processes of natural selection Evo-devo in appropriate study systems is providing the means to explore fully how phenotypic variation is generated by the processes intrinsic...

Are transposition events at the origin of the bilaterian Hox complexes

The genome sequences of two non-bilaterian animals, the cnidarians NematosteUa vectensis and Hydra magnipapillata, have been recently completed. These new data lead to the fascinating result that the complement of Hox genes in the cnidarian ancestor is considerably lower than that in the bilaterians, although the complexity of their genome is otherwise similar Technau et al. 2005 . Thus, there is a correlation between the radiation of the Bilateria and the expansion of the Hox complex. In the...

Chiparian Planula

Planula And Bilaterians

Bra. -cat. obi. chd gtc, bmp5-8 ash Figure 12.4 Comparative axial expression, in a simplified form, of key developmental genes between cnidarians planula larva, top left, and polyp, bottom left and bilaterians right . In the planula larva genes expressed asymmetrically along the directive axis 'D-V' axis are also depicted. A anterior AB aboral D dorsal O oral P posterior V ventral. 'D' and 'V' imply the likely, but still undefined, DV character of the directive axis in...

Conflicting hypotheses on the nature of megaevolution

Parasitic Platyhelminthes

Here is a question of the utmost importance for our understanding ofwhat has been called the 'big picture' of evolution Simpson 1944,1953 are the divergences that lead ultimately to high-level sister groups, such as those that would typically be labelled as orders, classes and phyla, qualitatively or quantitatively different from those that lead to low-level sister groups, such as races, species and genera In other words, is megaevolution more than just accumulated micro macro-evolution, or...